Xylene isomers, para-xylene, meta-xylene and ortho-xylene, are important intermediates which find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid, which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene, which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate with rapidly growing demand, but amounts to only 20 to 25% of a typical C8 aromatics stream. Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene (C7) is dealkylated to produce benzene (C6) or selectively disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered.
A current objective of many aromatics complexes is to increase the yield of xylenes and to de-emphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C9 and heavier aromatics with benzene and toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,365,104 discloses a process for modifying ZSM-5 type zeolite catalysts with sulfur-based treating agents in order to enhance para-selective catalyst properties based upon the molecular sieve.
U.S. Pat. No. 4,857,666 discloses a transalkylation process over mordenite and suggests modifying the mordenite by steam deactivation or incorporating a metal modifier into the catalyst.
U.S. Pat. No. 5,763,720 discloses a transalkylation process for conversion of C9+ into mixed xylenes and C10+ aromatics over a catalyst containing zeolites illustrated in a list including amorphous silica-alumina, MCM-22, ZSM-12, and zeolite beta, where the catalyst further contains a Group VIII metal such as platinum. Treatment to reduce aromatics loss by ring hydrogenation over such a catalyst includes sulfur exposure.
U.S. Pat. No. 6,060,417 discloses a transalkylation process using a catalyst based on mordenite with a particular zeolitic particle diameter and having a feed stream limited to a specific amount of ethyl containing heavy aromatics. Said catalyst contains nickel or rhenium metal.
U.S. Pat. No. 6,486,372 discloses a transalkylation process using a catalyst based on dealuminated mordenite with a high silica to alumina ratio that also contains at least one metal component.
U.S. Pat. No. 6,613,709 discloses a catalyst for transalkylation comprising zeolite structure type NES and metals such as rhenium, indium, or tin. The use of sulfur is disclosed, but Example 4 shows a sulfurization step (called sulphurization) that is only performed on a catalyst C2 containing nickel, which is described as ‘not in Accordance with the Invention’. Also, nothing is disclosed about the effect of sulfur on methane yield.
Many types of supports and elements have been disclosed for use as catalysts in processes to transalkylate various types of aromatics into xylenes, but the problem presented by high methane production associated with rhenium containing catalysts appears to have gone as yet unrecognized in the art. Applicants have found a solution with specific sulfur treatment of rhenium supported on solid-acid catalysts that increases yield of xylenes and decreases yield of undesired methane, which is associated with high metal hydrogenolysis activity. Avoidance of high metal hydrogenolysis activity becomes especially important under conditions of low total hydrogen partial pressure.